This invention relates to pulverizing and firing systems for solid fuels, and in particular to a mill recirculation system which while being operative for purposes of effecting the pulverization and subsequent firing of solid fuels in any form of structure that embodies a suitable type of combustion chamber, e.g., boilers, kilns, furnaces, air heaters, etc. is imbued with those features that serve to desirably characterize a direct fired system.
There are three basic types of solid fuel pulverizer firing systems in use today. These are the direct-fired system, the semi-direct fired system and the bin storage system. The simplest and most commonly used of these three systems is the direct-fired system. The nature of this latter system is such that solid fuel, e.g., wet coal, is fed in a suitable manner along with hot gases to a pulverizer. The solid fuel is simultaneously ground and dried within the pulverizer. The drying of the solid fuel is effected by the hot gases as the latter sweep through the pulverizer. The pulverizer that is utilized to accomplish the above may take the form of a hammermill, a ring-roll mill or a ball mill. As the hot gases sweep through the pulverizer they are cooled and humidified by means of the evaporation of the moisture contained in the solid fuel. Normally, a fan is utilized for purposes of removing the hot gases and the entrained fine solid fuel particles from the pulverizer. Moreover, usually this fan is located on the discharge side of the pulverizer and is operative to effect the delivery of the mixture of hot gases and entrained fine solid fuel particles to a burner.
The main advantages of the direct-fired system are simplicity, low cost and maximum safety. The potentially hazardous fine solid fuel particles go directly to the burner at high velocities, and thus are not given the opportunity to collect and possibly ignite spontaneously. Accordingly, the direct-fired system can be operated at the maximum temperatures that safety will allow. Further, in those instances wherein the pulverization of the solid fuel is effected by means of hammermills or ring-roll mills there is very little solid fuel present in the system at any given time. Therefore, should a fire occur in the system, it will be of relatively small size and as such is capable of being readily extinguished.
However, there is one major disadvantage associated with the employment of a direct-fired system. This consists of the fact that all of the hot gas, e.g., air, that is required for purposes of drying the solid fuel particles plus the air that infiltrates the pulverizer becomes primary air for the burner. Therefore, in those instances wherein the solid fuel particles are very wet more air is required for drying. Accordingly, the quantity of primary air thus forms a large percentage of the air which is required to support combustion. Further, in the case of pulverizers that take the form of hammermills and ring-roll mills, the amount of air that is required to flow therethrough in order for the pulverizer to operate at maximum capacity may be in excess of that required to dry the solid fuel particles. Lastly, the air which leaves the pulverizer is usually low in temperature and high in moisture. Unfortunately, though, the thermal efficiency of the burner is adversely affected when air that is low in temperature and/or high in moisture is utilized to support combustion in the burner.
Thus, to recapitulate, the mode of operation of a direct-fired system is such that all of the hot gas which is required to dry the solid fuel particles as well as that which is required to sweep the pulverizer for purposes of effecting the transport therethrough of the solid fuel particles operates also to effect the conveyance of the pulverized solid fuel to the combustion chamber of the burner wherein the solid fuel is fired. Moreover, since the conveying medium is usually air, the latter becomes part of the combustion air that is required to effect the burning, i.e., firing, of the solid fuel. Unfortunately, the hot gas, e.g., air, required to satisfy the drying, grinding, classifying requirements imposed thereupon by virtue of the nature of the operation of the pulverizer constitutes a relatively large quantity thereof and also is at a relatively low temperature. Both of these factors render the hot gas that flows through the pulverizer undesirable for use as combustion air in the burner. On the other hand, in most applications wherein a pulverizer is employed in conjunction with a burner to supply pulverized solid fuel thereto, there is an adequate amount of hot combustion air available, which has been recuperated from the exhaust gases of the system through the use of heat exchangers. Consequently, by utilizing the hot combustion air that has been recuperated from the exhaust gases of the system in lieu of the hot air that flows through the pulverizer, it is possible to improve the thermal efficiencies of the system and concomitantly thereby reduce the fuel consumption requirements thereof.
Turning next to a consideration of the second of the three types of firing systems referred to hereinbefore, i.e., that of the semi-direct fired system, the development thereof was occasioned principally by the desire to overcome the disadvantage of the direct-fired system which has been discussed above, while yet providing a system that would maintain the desirable safety and low cost features which are characteristic of a direct-fired system. Thus, in accord with the mode of operation of the aforesaid semi-direct fired system, the mixture of pulverized solid fuel particles and spent drying gases is conveyed through the action of a system fan to a cyclone collector whereat a separation thereof is effected. Namely, a portion of the spent drying gases is circulated from the cyclone collector back to the pulverizer whereat the recirculated spent drying gases are reheated by virtue of being mixed with high temperature fresh hot gases with which the pulverizer is being fed. The remainder of the spent drying gases that are received at the cyclone collector are vented. Desirably, the portion of the spent drying gases that is vented equals the weight of the fresh hot gases fed to the pulverizer, the amount of air that leaks into the pulverizer, and the water that is evaporated. Generally, under most conditions, the quantity of spent drying gases that is vented is considerably less than the total quantity that is required to flow through the pulverizer for purposes of effecting the efficient operation of the latter.
Continuing, the quantity of spent drying gases that is vented is then directed to the solids discharge area of the cyclone collector whereat the vented gases pick up the pulverized solid fuel particles and function to convey the latter in the form of a mixture of pulverized solid fuel particles and vented gases having a very high fuel to air ratio to the combustion chamber of the burner. The conveying vented gases, e.g., air, then become a very small percentage of the total amount of combustion air that is required to effect the firing of the pulverized solid fuel particles in the burner. The additional air necessary to support combustion is then introduced into the burner from the recuperator. That is, this additional air constitutes hot air which has been recuperated from the system's exhaust gases.
Finally, the remaining one of the three types of firing systems that has yet to be discussed herein is that of the bin storage system. In accord therewith, the hot gas flow circuit associated with the functioning of the pulverizer is totally divorced of the hot gas flow which the burner receives. More specifically, the mode of operation of the bin storage system is such that the mixture of pulverized solid fuel particles and spent drying gases is conveyed to a cyclone collector whereat the pulverized fuel particles are discharged into a storage bin and the drying gases are vented to a secondary collector and thence to the atmosphere. As required, quantities of pulverized solid fuel particles are removed from the storage bin along with a relatively small quantity of conveying air thereby maximizing the amount of heated recuperated air which can be employed as combustion air for purposes of firing the pulverized solid fuel particles in the burner. Accordingly, the bin storage system provides the highest thermal efficiency of the three firing systems that have been discussed herein, i.e., the direct-fired system, the semi-direct fired system and the bin storage system.
Insofar as a comparison of the three above-described firing systems is concerned, the increase in thermal efficiency which is achieved with the semi-direct fired system and the bin storage system is obtainable only at the expense of providing a system that has less desirable operating features and which is more complex. By way of exemplification in this regard, note is taken of the fact that pulverized solid fuel particles can pose a potential hazard insofar as the handling and storage thereof is concerned. Moreover, pulverized solid fuel particles are known to be susceptible to igniting spontaneously.
On the other hand, the main advantages of the direct-fired system are its simplicity, low cost, and safe mode of operation. These advantages stem principally from the fact that in accord with the mode of operation of the direct-fired system the potentially hazardous pulverized solid fuel particles are conveyed directly to the combustion chamber of the burner at relatively high velocities whereat they are fired. Consequently, problems associated with the handling and storage of the pulverized fuel particles are avoided. Likewise, with such a mode of operation there is no opportunity for the pulverized fuel particles to collect and subsequently spontaneously ignite.
As regards the semi-direct fired system, the latter has a less desirable mode of operation when compared to the aforereferenced direct-fired system in that the pulverized solid fuel particles upon entering the cyclone collector pass through both limits of the explosive range thereof as the hot gases are being separated therefrom. Therefore, the pulverized fuel particles become very sensitive to temperature and are susceptible to being ignited upon being exposed to system vent temperatures of a relatively high nature. Additionally, in the semi-direct fired system the cyclone collector is usually operated at a relatively high negative pressure whereas the line located therebeneath through which the pulverized solid fuel particles upon being discharged from the cyclone collector are conveyed to the combustor is usually at a very high positive pressure. Consequently, the valve which is utilized to discharge the pulverized fuel particles from the cyclone collector into the aforementioned conveying line operates at an extremely high differential temperature which produces rapid wearing of the valve. This wearing of the valve in turn gives rise to the occurrence of subsequent leakage of the conveying gas from the line into the cyclone collector. Furthermore, such leakage has an adverse effect on the operating efficiency of the cyclone collector and also can occasion a condition wherein a mixture of solid fuel particles and hot gases, which is of an explosive nature, is caused to be recycled back to the pulverizer.
When compared to the other two forms of firing systems and most particularly to the direct-fired system, the bin storage system is disadvantageously characterized in at least two significant respects. First, by virtue of the nature of the mode of operation of the bin storage system there exists a requirement that pulverized fuel particles be stored in a storage bin. It is a known fact, however, that pulverized solid fuel particles when stored can spontaneously ignite. Moreover, should such spontaneous ignition of the particles occur, the extinguishment and the removal of the ignited particles from the storage bin could be expected to present a problem. Thus, in an effort to minimize the extent of this problem, storage bins for storing such pulverized fuel particles have heretofore been sealed and pressurized with inert gas. Unfortunately, however, to do this is rather costly.
With further reference to the matter of the storage bin, ensuring that pulverized fuel particles are discharged therefrom at a uniform control rate can necessitate the employment in cooperative association with the storage bin of some type of means which is undesirably characterized both in terms of its complex construction and the fact that it is costly to provide. By way of exemplification in this regard, reference is had here to the fact that some forms of pulverized solid fuels such as pulverized coal have flow characteristics that are much like those of water whereas other forms of pulverized solid fuels such as pulverized bark and wood have a tendency to collect and effect a bridging of the discharge outlet of the storage bin thereby requiring the utilization of a further means that has the operative capability to negate this tendency of the pulverized fuel to collect and effect the bridging of the discharge outlet of the storage bin.
The second notable disadvantage of the bin storage system involves the gaseous discharge that occurs therefrom to the atmosphere. Namely, since cyclone collectors are known to be less than one hundred percent efficient in removing all of the particulate matter from the mixture of solid fuel particles and conveying gases that is received thereby, particulate matter is emitted along with the gas that is exhausted therefrom to the atmosphere. Further, it is possible that the extent of such particulate matter emission may be such as to run afoul of the air pollution requirements that are in effect in the jurisdiction in which the bin storage system is being employed. In addition, in those instances wherein a high pressure drop wet scrubber is utilized for purposes of effecting the removal of particulate matter from the gas stream, a further problem may be posed. More specifically, the nature of the mode of operation of a high pressure drop wet scrubber is such that relatively large quantities of water are required to accomplish the removal of the particulate matter to the extent desired. However, the need for such large quantities of water creates a disposal problem of its own since the water effluent from the scrubber may contain up to one to two percent of pulverized solid fuel particles. Usually, these solid fuel particles are required to be removed from the water effluent before the latter can be discharged into a local sewage system.
Accordingly, the type of secondary collector which is most commonly used with a bin storage system is that of a cloth bag dust collector. The latter which is often referred to as a "baghouse", operates to effectively recover the particulate matter which is contained in the gases that are to be vented from the system to the atmosphere, as well as to effect the return of the recovered particulate matter to a suitable location. However, there are hazards associated with the use of a cloth bag dust collector to recover particulate matter from vent gases that are at relatively high temperatures. Namely, the particulate matter which enters the dust collector is of an extremely fine nature and thus can very easily spontaneously ignite if the particulate matter is not kept in a constant state of motion. Small upward excursions in the temperature of the gases that contain the particulate matter, which is sought to be recovered through the use of the dust collector, can be sufficient to cause the particulate matter to spontaneously ignite.
A need has thus been evidenced for a new and improved firing system that would be advantageously characterized by the fact that the mode of operation thereof enables a more desirable fuel/air ratio to be established at the burner, while yet providing a firing system which retains the advantages of a direct-fired system insofar as simplicity, low cost and safety are concerned. More specifically, such a new and improved firing system has been sought wherein the more desirable fuel/air ratio that is established thereby at the burner is accomplished as a consequence of causing the recirculation back to the pulverizer of a portion of the gases leaving the classifier.
It is, therefore, an object of the present invention to provide a new and improved form of firing system of the type that is operative for purposes of effecting the pulverization of solid fuels followed by the firing thereof.
It is another object of the present invention to provide such a firing system which is in the nature of a direct fired system.
It is still another object of the present invention to provide such a direct fired system which possesses the advantages of a direct fired system insofar as simplicity, low cost and safety are concerned.
A further object of the present invention is to provide such a direct fired system which is further advantageously characterized by the fact that in accord with the mode of operation thereof a more desirable fuel/air ratio is established at the burner.
A still further object of the present invention is to provide such a direct fired system wherein the establishment of a more desirable fuel/air ratio at the burner is accomplished as a consequence of causing the recirculation back to the pulverizer of a portion of the gases that exit from the classifier and without requiring the use within the system of a cyclone collector or a discharge valve.